[SI-LIST] Crystal osciiator not oscillating

 
Hi Gurus, 

  

           In newly designed board, I am facing a problem of crystal
oscillator not oscillating. This crystal is placed in between the XTAL
&EXTALpin of the microcontroller. Also the load capacitors of 15pF
populated.

           uC has internal inverter. Input and output of the inverter has
1MegaOhm resistor &series resistance of 475E at the output of the inverter. 

           After so many trials on the boards I found out that there are two
cases in which crystal start oscillating –

  

1)       Lowering the value of the external load capacitors form 15pF to
1pF.

2)       By keeping the load capacitance value same as 15pF,  a 100K
externalresistor between the XTAL &EXTAL pin of the microcontroller,
population starts oscillations. 

  

What will be the reason because of that crystal is not able to oscillate? 

  

Thanks &Regards, 

Manish Narayane 

HCL Tech, chennai, India 

   

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